Binary semaphore creation and usage in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a binary semaphore.

FreeRTOS

SemaphoreHandle_t xSemaphore = [1]();

Drag options to blanks, or click blank then click option'

AxSemaphoreCreateBinary

BxSemaphoreCreateMutex

CxQueueCreate

DvSemaphoreCreateBinary

Attempts:

3 left

💡 Hint

Common Mistakes

Using xSemaphoreCreateMutex instead of xSemaphoreCreateBinary

Using vSemaphoreCreateBinary which is deprecated

Confusing queue creation with semaphore creation

The function xSemaphoreCreateBinary() creates a binary semaphore in FreeRTOS.

2fill in blank

medium

Complete the code to take (acquire) the binary semaphore with a block time of 100 ticks.

FreeRTOS

if (xSemaphoreTake(xSemaphore, [1]) == pdTRUE) {
    // Critical section
}

Drag options to blanks, or click blank then click option'

A0

BportMAX_DELAY

C100

DpdFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using pdFALSE which is not a valid block time

Using portMAX_DELAY when the question asks for 100 ticks

Using 0 which means no wait

The second parameter of xSemaphoreTake() is the block time in ticks. Using 100 means it waits up to 100 ticks to take the semaphore.

3fill in blank

hard

Fix the error in the code to give back (release) the binary semaphore.

FreeRTOS

xSemaphore[1](xSemaphore);

Drag options to blanks, or click blank then click option'

AGive

BTake

CGiveFromISR

DRelease

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Take' instead of 'Give' to release the semaphore

Using 'Release' which is not a FreeRTOS function

Using 'GiveFromISR' outside an ISR context

The correct function to release a binary semaphore is xSemaphoreGive().

4fill in blank

hard

Fill both blanks to create a binary semaphore and check if creation was successful.

FreeRTOS

SemaphoreHandle_t xSemaphore = [1]();
if (xSemaphore [2] NULL) {
    // Semaphore created successfully
}

Drag options to blanks, or click blank then click option'

AxSemaphoreCreateBinary

B==

C!=

DxSemaphoreCreateMutex

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' to check for successful creation

Using xSemaphoreCreateMutex instead of xSemaphoreCreateBinary

Use xSemaphoreCreateBinary() to create the semaphore and check if it is not equal to NULL to confirm success.

5fill in blank

hard

Fill all three blanks to take the semaphore with no wait and then give it back.

FreeRTOS

if (xSemaphore[1](xSemaphore, [2]) == pdTRUE) {
    xSemaphore[3](xSemaphore);
}

Drag options to blanks, or click blank then click option'

ATake

B0

CGive

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using portMAX_DELAY instead of 0 for no wait

Mixing up Take and Give function names

Not matching the block time parameter correctly

Use xSemaphoreTake() with block time 0 to try taking the semaphore without waiting, then release it with xSemaphoreGive().

Practice

1. What is the primary purpose of a binary semaphore in FreeRTOS?

easy

A. To store multiple values for task communication

B. To signal and control access to shared resources between tasks

C. To create multiple tasks automatically

D. To allocate memory dynamically

Solution

1. Step 1: Understand the role of binary semaphores

   Binary semaphores are used to signal availability or control access to resources between tasks.

2. Step 2: Compare options with this role

   Only To signal and control access to shared resources between tasks describes signaling and controlling access, which matches the purpose of binary semaphores.

3. Final Answer:

   To signal and control access to shared resources between tasks -> Option B

4. Quick Check:

   Binary semaphore = resource signaling [OK]

Hint: Binary semaphore controls resource access between tasks [OK]

Common Mistakes:

• Confusing binary semaphore with counting semaphore
• Thinking it stores multiple values
• Assuming it creates tasks

2. Which of the following is the correct way to create a binary semaphore in FreeRTOS?

easy

A. xSemaphoreCreateBinary();

B. xSemaphoreCreateCounting(1, 0);

C. xSemaphoreCreateMutex();

D. xSemaphoreCreateBinary(1);

Solution

1. Step 1: Recall the function to create a binary semaphore

   The correct function is xSemaphoreCreateBinary() with no parameters.

2. Step 2: Check each option

   xSemaphoreCreateBinary(); matches the correct function. xSemaphoreCreateCounting(1, 0); creates a counting semaphore, C creates a mutex, and D is invalid syntax.

3. Final Answer:

   xSemaphoreCreateBinary(); -> Option A

4. Quick Check:

   Create binary semaphore = xSemaphoreCreateBinary() [OK]

Hint: Use xSemaphoreCreateBinary() with no arguments [OK]

Common Mistakes:

• Using counting semaphore creation instead
• Adding parameters to xSemaphoreCreateBinary()
• Confusing mutex creation with binary semaphore

3. Consider the following FreeRTOS code snippet:

SemaphoreHandle_t xBinarySemaphore = xSemaphoreCreateBinary();

void Task1(void *pvParameters) {
    if (xSemaphoreTake(xBinarySemaphore, portMAX_DELAY) == pdTRUE) {
        // Critical section
    }
}

void Task2(void *pvParameters) {
    xSemaphoreGive(xBinarySemaphore);
}

What will happen when Task1 runs before Task2?

medium

A. Task1 will enter the critical section immediately

B. Task1 will skip the critical section

C. Task1 will cause a runtime error

D. Task1 will block indefinitely until Task2 gives the semaphore

Solution

1. Step 1: Analyze xSemaphoreTake with portMAX_DELAY

   Task1 calls xSemaphoreTake with portMAX_DELAY, so it waits indefinitely for the semaphore.

2. Step 2: Consider initial semaphore state and Task2 action

   The semaphore is created but not given initially, so Task1 blocks until Task2 calls xSemaphoreGive.

3. Final Answer:

   Task1 will block indefinitely until Task2 gives the semaphore -> Option D

4. Quick Check:

   xSemaphoreTake with portMAX_DELAY blocks until given [OK]

Hint: xSemaphoreTake with portMAX_DELAY waits until given [OK]

Common Mistakes:

• Assuming semaphore is initially available
• Thinking Task1 runs critical section immediately
• Confusing blocking with error

4. Identify the error in this FreeRTOS binary semaphore usage:

SemaphoreHandle_t xBinarySemaphore = xSemaphoreCreateBinary();

void Task(void *pvParameters) {
    if (xSemaphoreTake(xBinarySemaphore, 0) == pdTRUE) {
        // Do work
        xSemaphoreGive(xBinarySemaphore);
    }
}

medium

A. xSemaphoreCreateBinary requires a parameter

B. xSemaphoreGive should be called before xSemaphoreTake

C. Semaphore is never given before take, so take always fails

D. xSemaphoreTake timeout cannot be zero

Solution

1. Step 1: Check initial semaphore state after creation

   xSemaphoreCreateBinary creates a semaphore that is initially empty (not given).

2. Step 2: Analyze xSemaphoreTake with zero timeout

   With zero timeout, xSemaphoreTake returns immediately. Since semaphore is empty, it returns pdFALSE, so the if block never runs.

3. Final Answer:

   Semaphore is never given before take, so take always fails -> Option C

4. Quick Check:

   Binary semaphore empty at creation, take with zero timeout fails [OK]

Hint: Binary semaphore must be given before take succeeds [OK]

Common Mistakes:

• Assuming semaphore is initially given
• Thinking zero timeout means wait forever
• Confusing function parameters

5. You want to synchronize two tasks so that Task B only runs after Task A signals completion using a binary semaphore. Which sequence correctly implements this?

hard

A. Task A calls xSemaphoreGive() after work; Task B calls xSemaphoreTake() with portMAX_DELAY before work

B. Task B calls xSemaphoreGive() before work; Task A calls xSemaphoreTake() after work

C. Both tasks call xSemaphoreTake() simultaneously; semaphore is given once at start

D. Task A and Task B both call xSemaphoreGive() without taking

Solution

1. Step 1: Understand synchronization with binary semaphore

   Task B must wait for Task A's signal, so Task B should take the semaphore and block until given.

2. Step 2: Match sequence to this logic

   Task A gives the semaphore after finishing work; Task B takes it with portMAX_DELAY to wait indefinitely.

3. Final Answer:

   Task A calls xSemaphoreGive() after work; Task B calls xSemaphoreTake() with portMAX_DELAY before work -> Option A

4. Quick Check:

   Give after work, take before work = sync tasks [OK]

Hint: Give semaphore after work; take before work to sync tasks [OK]

Common Mistakes:

• Reversing give and take roles
• Both tasks giving without taking
• Taking without giving semaphore